Chances are you've already been exposed to it. One doesn't get exposed to mono per se. Mono, short for mononucleosis, is a blood condition which occurs usually as a result of infection with the Epstein-Barr virus. However, not everyone who is exposed to EBV gets mono. Most adults by the time they're 40 have been exposed to EBV at some point in their lives, but the virus is dormant, not causing any symptoms.

If your boyfriend had mono and you've never been exposed to it before, he could have passed it to you any time from before he became symptomatic, even to after for a few weeks. However, as I mentioned before, that does not mean that you are necessarily going to get mono. In fact, chances are you won't.